Semiconductor chips or wafers are used in many applications, including as processor chips for computers, and as integrated circuits and as flash memory for hand held computing devices, wireless telephones, and digital cameras. Regardless of the application, it is desirable that a semiconductor chip hold as many circuits or memory cells as possible per unit area. In this way, the size, weight, and energy consumption of devices that use semiconductor chips advantageously is minimized, while nevertheless improving the memory capacity and computing power of the devices.
A common circuit component of semiconductor chips is the transistor. In ULSI semiconductor chips, a transistor is established by forming a polysilicon gate on a silicon substrate, and then forming a source region and a drain region side by side in the substrate beneath the gate by implanting appropriate dopant materials into the areas of the substrate that are to become the source and drain regions. The gate is insulated from the source and drain regions by a thin gate oxide layer, with small portions of the source and drain regions, referred to as "extensions", extending toward and virtually under the gate. This generally-described structure cooperates to function as a transistor.
As discussed above, conventional transistor gates are made of polysilicon. As recognized herein, adding germanium to the polysilicon to render a transistor gate composition having the pseudo-chemical formula poly-Si.sub.1-x Ge.sub.x results in certain advantages over conventional polysilicon gates. One advantage is that a transistor with a poly-Si.sub.1-x Ge.sub.x gate structure has a "variable work function". Stated differently, the threshold voltage of the transistor (a critical transistor performance characteristic) can be established by appropriately establishing the mole fraction of the germanium in the gate. This is a better way to establish a desired threshold voltage than by adjusting the dopant concentration in the channel region (i.e., the region between the source and drain under the gate), as currently must be done in polysilicon gate devices, because adjusting the dopant concentration in channel regions can cause unwanted short-channel effect and degradation of channel carrier mobility. Both short-channel effect and degradation of channel carrier mobility degrade the performance of the transistor.
Furthermore, when germanium is used in a polysilicon transistor gate, the dopant can be activated using lower activation temperatures during fabrication. Looked at another way, at a given temperature more dopant per unit time can be activated in poly-Si.sub.1-x Ge.sub.x gate material than in polysilicon gate material. Consequently, two phenomena that degrade transistor performance--gate sheet resistance and gate depletion effect--are reduced in poly-Si.sub.1-x Ge.sub.x gates vis-a-vis polysilicon gates. Moreover, in general it is desirable to use relatively low temperatures, including low dopant activation temperatures, during semiconductor fabrication to reduce process cost and complexity and to reduce the risk of temperature-induced damage to chip components.
With the above in mind, the present invention recognizes the desirability of incorporating germanium into polysilicon transistor gates. However, the present invention further recognizes that a relatively high dose of germanium in the polysilicon must be implanted to achieve the above-mentioned advantages. Indeed, germanium doses in the range of 8.times.10.sup.16 germanium atoms per square centimeter to 1.times.10.sup.17 germanium atoms per square centimeter are required. As understood by the present invention, to achieve such high germanium doses, the implantation of germanium into the gate requires many hours, thereby prolonging fabrication time and correspondingly reducing production throughput. Fortunately, the present invention addresses the problem of achieving high germanium doses in polysilicon transistor gates while minimizing fabrication time and, thus, improving production throughput.